Wide area networks, so called WANs, are used as a transmission medium for connecting local area networks, so called LANs. An article by G. Parr et al. ("A Proposed Protocol Improvement for ATM Cell Processing within SDH Multiplexers", ConneXions Vol. 10, No. 11, November 1996, pp. 14-24) describes the fact that the data packets are first transmitted to an interworking unit, which has a routing table for its local nodes. Outgoing data packets whose addresses are not contained in the routing table are converted by an ATM access unit into ATM cells (ATM: asynchronous transport mode) and are conveyed by an ATM switch into suitable virtual channels within virtual paths of the ATM-WANs. From there, the ATM cells travel to a synchronous multiplexer, which operates in accordance with the SDH or SONET recommendations and standards of the ITU and ANSI, and are packed into virtual containers and synchronous transport modules for transmission. So that all of the data packets do not have to be unpacked again at each SDH node, data bytes are written into the path overhead of the virtual containers, which indicate whether a virtual container must be unpacked again in the next SDH node. Disadvantages of this method are that a number of costly devices are required for the ATM network and that the transmission times are relatively long.
Today, data packets are mostly structured in accordance with the internet protocol (IP) and have a source IP address and a target IP address. In an article from the Ipsilon Networks Co. ("IP Switching: The Intelligence of Routing, the Performance of Switching", Ipsilon Technical White Paper on IP Switching, February 1996, which can be accessed on the internet at http://www.ipsilon.com/productinfo/wp-ipswitch.html), a device (IP switch) is disclosed, which combines the function of an IP router and an ATM switch. It is in a position to identify longer sequences of data packets with the same source and target address, so-called flows and to establish device-internal connections suitable for them. As a result, the throughput of data packets through the IP switch increases and the average switching time is reduced. The costs for a transmission system with this kind of IP switches are relatively high.
In RFC1619 (W. Simpson, Request for Comments 1619, Internet Engineering Task Force, Network Working Group, May 1994), for point-to-point connections, the proposal is made to pack data packets by means of a frame structure, which is called a synchronous payload envelope (SPE), directly into synchronous transport modules (STM-N) from SDH or synchronous transport signals (STS-N) from SONET. However, this has the disadvantage that all of the transport modules must be unpacked in each network node of the synchronous data transmission network, which on the one hand, puts a strain on the network nodes and can lead to stoppages, and on the other hand, increases the transmission times.